The present invention relates to meniscus coating methods and apparatus in which the need to use volatile organic solvents to carry or dissolve the coating material is obviated by the use of a carbon dioxide liquid that contains the coating component.
There are three forms of meniscus coating processes which are commonly grouped under the term xe2x80x9cfree meniscus coatingxe2x80x9d: Withdrawal processes, drainage processes, and continuous processes. Many other coating processes use a meniscus to produce films on the substrate to be coated. These include roll coating, blade coating, and slot coating.
Withdrawal coating (often referred to as dip coating) is the most common free meniscus technique used in both laboratories and industry because of its simplicity and cost. Continuous coating is often desirable because of higher output, but the complicated engineering involved often prevents it from being utilized. Drainage is based upon the same principles as withdrawal and is advantageous when space is limited since it requires no mechanical lifting mechanism. See, e.g., C. Brinker et al., in Liquid Film Coating, 673-708 (S. Kistler and P. Schweizer eds. 1997).
Free meniscus coating is a solvent intensive process and accounts for a considerable use of environmentally undesireable solvents. Accordingly, there is a need for new free meniscus coating methods and apparatus that reduce or eliminate the use of solvents such as VOCs and the use of solvents such as CFC, HCFC, HFC, or PFC solvents, as well as aqueous solvents.
A method of coating a substrate having a surface portion comprises immersing a surface portion of a substrate in a first phase, the first phase comprising carbon dioxide and a coating component such as a polymer; and then withdrawing the substrate from the first phase into a distinct second phase so that said coating component is deposited on said surface portion. In general, the first phase is a liquid or a supercritical fluid (with supercritical fluids preferred for polymer melts), and the second phase is a gas. The withdrawal step is typically followed by the step of separating the carbon dioxide from the coating component (e.g., by evaporation, venting, heating, etc.) so that the coating component is retained as a coating layer formed on the surface portion.
A second aspect of the invention is an apparatus useful for coating a substrate, comprising: a high pressure carbon dioxide supply vessel; a high pressure cell coating vessel connected to the carbon dioxide supply vessel and configured to contain separate and distinct first and second phases therein, the first phase comprising liquid or supercritical carbon dioxide; a holding device for engaging a substrate to be coated in the coating vessel; and a drain system, a batch or continuous mechanical withdrawal assembly, or other withdrawal means operatively associated with said holding device for removing a surface portion of said substrate from said first phase to said second phase in said coating vessel.
A third aspect of the present invention is an apparatus useful for coating a substrate, comprising: a high pressure carbon dioxide supply vessel; a high pressure coating vessel connected to the carbon dioxide supply vessel for containing a liquid or supercritical fluid comprising carbon dioxide and a coating component; a roller assembly, conveyor line, moving table or other such substrate supply means for moving a substrate to be coated in a direction of travel; a feed line connected to the coating vessel and configured to deposit the liquid or supercritical fluid on said substrate at a predetermined location along the direction of travel; and a blade, knife, roll, or other such metering means operatively associated with the supply device for metering the amount of said liquid or supercritical fluid deposited on the substrate.
The foregoing and other objects and aspects of the present invention are explained in greater detail in the drawings herein and the specification set forth below.